Oxygen isotopic composition of fossil horse tooth phosphate as a record of continental paleoclimate

Oxygen isotopic, elemental, and X-ray data are presented for a suite of 24 fossil horse teeth from Nebraska ranging in age from 18.2 to 8.5 Ma, to test the use of δ¹⁸O of enamel phosphate (δ¹⁸O_P04) as a quantitative record of continental climate. Modern equid teeth were analyzed to estimate a relationship between δ⁸O_P04 and environment water. Multiple samples of seven different fossil species from Burge Quarry, a ∼ 12 Ma attritional fossil deposit, indicate the diagenetic overprints exist but can be detected by decreased P concentration and increased crystallinity relative to modern enamel. Isotopic variation for the pristine samples from Burge Quarry is ±1.5% (1σ, n=9), which may represent the resolution of the procedure within a stratigraphic horizon. There are no apparent correlations with body size, hypsodonty, or phylogeny. A range of 7%0 in δ¹⁸O_P04 occurs over the 10 m.y. interval. A trend towards depleted δ¹⁸O_P04 of about 4% corresponds to a depletion of up to 6%0 in δ¹⁸O of precipitation between 18.2 and 8.5 Ma, but the range of variation of Burge is large relative to the climate signal. Our results demonstrate that δ¹⁸O_P04 should be useful in quantitatively reconstructing Cenozoic continental paleoclimate on 10⁶-year timescales. Isotopic variation due to taphonomic bias and the terrestrial rock record will likely obscure higher-order climate signals.